Patent GB-864548-A (GB)
Title: Improvements in hydraulic control systems for agricultural tractors
IPC: A01B63/114
Filed: 1957-06-26
Abstract: 864,548. Fluid-pressure servomotor control systems. SOC. INDUSTRIELLE GENERALE DE MECANIQUE APPLIQUEE S.I.G.M.A. June 26, 1957 [July 2, 1956], No. 11083/60. Divided out of 20185/57. Class 135. [Also in Group XXXI] An hydraulic control system for an agricultural implement pivotally mounted on a tractor comprises a jack 8, Fig. 2, interposed between the implement and the tractor, a liquid tank 13, a continuously operating pump 14, a valve 22 for connecting a delivery conduit 17 and the jack to a discharge conduit 24, and a valve 35 rigidly connected to a control member 36 for throttling the discharge conduit 24 to cause a part of the implement load to be transferred to the tractor, a valve 42 or other means being provided upstream of the valve 35 to limit the pressure to a value lower than that necessary to lift the implement. With the valve 22 in the position shown the chambers 11 and 12 of the jack are connected through conduits 18a, 19a and the valve 22 to conduits 24 and 25, respectively, leading to the tank 13 from which the pump 14 draws its supply. The conduit 24 is also connected to the part of conduit 17 between the valve 22 and the pump output and contains the valve 35. When the latter is moved to the right by the member 36 it throttles the flow from the pump 14 to increase the pressure in the chamber 11 and cause a portion of the load of the implement to be transferred to the rear wheels of the tractor to prevent skidding. The valve 42 opens at a pressure which is lower than that necessary to lift the implement and that to which a relief valve 21 in the pump output is set. The valve 35 has two portions of different diameters, D and d, to provide a reaction at the lever 35 which is substantially proportional to the pressure in a chamber 34 and may have a conical portion or triangular notches 38 to provide a gradual throttling effect. It may be formed as an extension on the valve 22 which is then moved to the right of the position shown to produce the throttling action. When the valve 22 is moved to the left of the position shown, the conduits 18a, 19a are closed and the pump may be connected to one or other of the conduits 18, 19 through the valve 15, the other conduit being connected through the valve and a conduit 20 to the tank 13. The valve 15 has a third position in which the implement is held at a desired level. The conduits 18a, 19a may be closed by spring-loaded valves operated through push rods by cam surfaces on the valve 22. A single-acting jack may be used and the valve 15 may be constructed as described in Specification 783,219 or 832,516. In Figs. 9 and 11, the main valve 15 consists of a follow-up device which has a valve element 55 connected by links 57, 58, 61 to a hand lever 16 and to the piston 8 of the jack. When the lever is moved the link 58 swings about a pivot 60 and the element allows fluid to flow to one or other side of the piston 8 until the movement of the piston restores the element 55 to neutral position by swinging the link 58 about a pivot 56. The flow of fluid to either side. of the piston opens a spring-loaded valve. 74 or 75 on the opposite side. To obtain load transfer, an auxiliary valve 63 is moved to the position shown in which flow from the pump conduit 17 is throttled at 38, Fig. 11, to raise the pressure in conduits 18a, 19a and cause a thrust on piston 8 due to the difference in area of its two faces, a calibrated orifice 70 providing an alternative path to the tank 13 limits the rise in pressure. The orifice may be provided in the valve body, for example, inside projection 68a. Rotary valves may be used. Specification 807,180 also is referred to.